Oxynitride glass-ceramics

ABSTRACT

The instant invention is directed to the production of glass-ceramic articles from thermally crystallizable glass compositions containing, by weight, about 40-85% SiO 2  and 2.5-17% N as basic constituents and utilizing such modifiers as the alkali metals, the elements of Groups IIA and IIB of the Periodic Table, boron, and aluminum to obtain practical glass-forming compositions. In general, at N contents greater than about 3.5%, the predominant crystal phase developed in situ will customarily comprise &#34;nitrogen mullite&#34;, silicon oxynitride (Si 2  ON 2 ), and/or β&#39;--Si 3  N 4  solid solution. The crystal phase which has been denominated in the literature as &#34;nitrogen-mullite&#34; has a lath-like morphology and, hence, is useful in reinforcing the crystalline body. Where the predominant crystal phase comprises a silicate, it is believed that nitrogen is present in some manner within the silicate crystal structure. The parent thermally crystallizable glasses appear to be self-nucleating, i.e., the addition of a conventional nucleating agent such as TiO 2 , ZrO 2 , or SnO 2  is not required to cause crystallization in situ. It is postulated that N may function as a nucleating agent.

This is a division of co-pending application Ser. No. 843,872, filedOct. 20, 1977, which is a division of Ser. No. 735,313, filed Oct. 26,1976, now U.S. Pat. No. 4,070,198, issued Jan. 24, 1978, which in turnis a continuation-in-part of Ser. No. 664,263, filed Mar. 5, 1976, nowabandoned.

BACKGROUND OF THE INVENTION

The field of glass-ceramics had its genesis in U.S. Pat. No. 2,920,971.As is explained therein, a glass-ceramic article is produced through thecontrolled crystallization in situ of a glass body. The method ofmanufacture contemplates three fundamental steps. First, a glass-formingbatch, commonly containing a nucleating agent, is melted. Second, thismelt is simultaneously cooled to an essentially crystal-free glass andan article of a desired configuration shaped therefrom. Third, the glassarticle is subjected to a particularly-defined heat treatment to causethe glass to crystallize in situ. In general, the heat treatment will beconducted in two steps. Thus, the glass article will be heated to atemperature somewhat above the transformation range to initiate thedevelopment of submicroscopic nuclei therein. Thereafter, the articlewill be heated to a higher temperature, frequently above the softeningpoint of the glass, to cause the growth of crystals on the nuclei.

Inasmuch as the crystallization results from the substantiallysimultaneous growth on countless nuclei dispersed through the glassbody, a glass-ceramic article will normally be highly crystalline, i.e.,greater than 50%, crystalline, and the individual crystals will berelatively uniformly fine-grained and homogeneously dispersed, butrandomly oriented, throughout a residual glassy matrix. The residualglassy matrix will customarily comprise but a small portion of theglass-ceramic article, and the composition thereof will be far differentfrom that of the parent or original glass since the components making upthe crystal phase will have been removed therefrom. It follows, then,that the physical properties demonstrated by the glass-ceramic articlewill normally be quite dissimilar to those exhibited by the parent glassbut will, instead, more nearly approach those of the predominant crystalphase contained therewithin.

Because glass-ceramics are produced through the crystallization in situof glass articles, forming methods conventional in the glassmaking artare equally suitable here. This factor has permitted articles of complexshapes to be fashioned in accordance with customary glass formingtechniques. Such shapes can thereafter be heat treated to secure thedesired fine-grained, highly-crystalline body. Furthermore, because ofthe residual glassy matrix, a glass-ceramic article will be non-porousand free from voids.

U.S. Pat. No. 2,920,971 provides a more complete discussion of thetheoretical considerations, as well as the practical aspects, involvedin the production of glass-ceramic articles and specific reference ismade to that patent for such discussion. The patent also describes thecustomary microstructure of a glass-ceramic as consisting offine-grained crystals homogeneously, but randomly oriented, in aresidual glassy-matrix, the crystals comprising at least 50% by weightof the article.

SUMMARY OF THE INVENTION

The instant invention is concerned with the production of glass-ceramicarticles from thermally crystallizable precursor glasses havingcompositions containing SiO₂ and nitrogen as base components, wherebynitrogen is believed to be present as the nitrogen ion N⁻³. The SiO₂ andnitrogen, expressed as N, comprise at least 50% by weight of thecomposition and are present in about 40-85% SiO₂ and 2.5-17% N. The N isincorporated as a network ion rather than as a gaseous species. Whereless than about 2.5% is included in the composition, its presence haslittle substantive effect on the overall properties of the parent glassand resulting glass-ceramic. The remainder of the composition consistsof network formers and/or modifiers (M_(x) O_(y)) which are added toenable glass bodies to be formed through conventional glassmakingpractices. Such additions include, inter alia, the alkali metal oxides,the oxides of the elements of Groups IIA and IIB of the Periodic Table,B₂ O₃, and Al₂ O₃.

The identity of the crystal phases developed in situ within the articlesis influenced by the amount of nitrogen present in the composition.Thus, where the nitrogen content is relatively low the predominantcrystal phase observed will commonly be identified through X-raydiffraction analysis as a silicate. Since chemical analysis hasindicated the presence of nitrogen in the crystal phase, it is surmisedthat nitrogen has, in some manner, entered into the structure of thesilicate crystal. Where the nitrogen content is high, the predominantcrystal phase, as identified via X-ray diffraction analyses, willcustomarily comprise at least one of the following nitride-containingsolid solutions: "nitrogen-mullite", silicon oxynitride (Si₂ ON₂), andβ'-Si₃ N₄.

Discussions of those nitrogen-containing solid solutions are providedin:

"A Preliminary Investigation of the Reaction of Alpha-Silicon Nitridewith Metakaolin", S. Wild, appearing in pages 309-318 of SpecialCeramics 6, Proceedings of the Sixth Symposium on Special Ceramics heldby the British Ceramic Research Association, July 9-11, 1974, edited byP. Popper and

"The Production of High-Temperature, High-Strength Nitrogen Ceramics",K. H. Jack, appearing in pages 265-286 of Ceramics for High-PerformanceApplications. Proceedings of the Second Army Materials TechnologyConference held at Hyannis, Massachusetts, November 13-16, 1973,published in cooperation with the Metals and Ceramics InformationCenter, Columbus, Ohio, editors John J. Burke, Alvin E. Gorum, R. NathanKatz

The practical glass-forming region of the simple binary SiO₂ -N isessentially non-existent. At least 15% and, preferably, at least 20% byweight of network formers and/or modifiers (M_(x) O_(y)) will beincluded in the composition to enable glasses to be obtained utilizingconventional glass production techniques. Therefore, the practicalmaximum SiO₂ content will generally range about 80% by weight. One ofthe most desirable additions is Al₂ O₃ which not only assists inpractical glass formation but also imparts good chemical durability tothe glass. However, the practical glass-forming region of the simpleternary SiO₂ -Al₂ O₃ -N is quite small, viz., about 55-70% SiO₂, 20-30%Al₂ O₃, and 3.5-15% N. Those ranges of glasses are illustrated in theternary composition diagram comprising FIG. 8. Nevertheless, theaddition of modifying ions can greatly enlarge the composition area inwhich essentially crystal-free glass bodies can be formed. The mostdesirable additions appear to be the alkali metals and the elements fromGroups IIA and IIB of the Periodic Table, with other useful additionsincluding, but not limited to, such elements as manganese, yttrium,lanthanum, and zirconium. The Group IIA elements can be added in amountsup to about 45% on the oxide basis. In general, the modifiers areconsidered to be substituted for SiO₂ in the base ternary system andthat the overall range of operable compositions will be included withinthe quaternary field, to yield a preferred range of compositions,expressed in weight percent, of about 40-80% SiO₂, 10-40% Al₂ O₃,3.5-17% N, and 5-25% M_(x) O_(y), wherein M_(x) O_(y) consists of theoperable network forming and/or modifying ingredients other than Al₂ O₃.In general M_(x) O_(y) will desirably consist of up to about 20% of theoxides of the Groups IIA and IIB elements and/or up to about 12% Li₂ O.

Prior Art

As was observed above in the discussion of nitride-containing solidsolutions, considerable research has been undertaken involving thosematerials in the field of ceramics. The activity in the field of glasshas been much less and has principally contemplated the passing ofgaseous nitrogen through a melt of molten glass which has resulted in asmall amount of nitrogen being dissolved and retained in the glass. U.S.Pat. No. 3,582,307 is illustrative of that practice. In that patent aglass-forming batch for an alkali metal aluminoborosilicate glass andalso containing carbon was melted and nitrogen bubbled through the meltwhile an atmosphere of nitrogen surrounded the melt. No mention is madeof heat treating the resultant glass to cause crystallization in situ toa glass-ceramic article and the amount of nitrogen dissolved in theglass is necessarily small, i.e., less than 2.5% N by weight.

DESCRIPTION OF PREFERRED EMBODIMENTS

Silicon nitride (Si₃ N₄) has been viewed with considerable interest inrecent years as a material with a great potential for many hightemperature engineering applications. The physical properties exhibitedby the material, viz., high strength, extraordinary resistance toabrasion, high decomposition temperature, excellent thermal shockresistance, and resistance to corrosive environments, to name but a few,would seem to recommend its unqualified utility for countless hightemperature applications. Unfortunately, however, fabricating shapes ofthe material has met with severe difficulties, thus sharply limiting itspractical utility. Inasmuch as Si₃ N₄ is a covalently-bonded solid,similar in that respect to diamond, it has proved impossible to sinterpure Si₃ N₄ to maximum density.

Silicon nitride exists in two crystal modifications, i.e., the alphaform and the beta form. The beta form is a typical covalent solid builtof SiN₄ tetrahedra with three tetrahedra sharing one corner, as is thecase in phenacite (Be₂ SiO₄) or in willemite (Zn₂ SiO₄). In contrast,the alpha form is believed to consist of an oxynitride with about oneout of every 30 nitrogen atoms being replaced by oxygen. The chargeimbalance is compensated for through the creation of cation vacancies.

Upon realizing that alpha-Si₃ N₄ is a defect structure wherein a fewnitrogen atoms are replaced by oxygen atoms, we conjectured that itmight be possible to replace more nitrogen atoms in the crystal latticewith oxygen atoms. In undertaking this project, we utilized principlessimilar to those employed for replacing a higher valency ion by a lowervalency ion in mineral silicates, e.g., by replacing Si⁺⁴ with Al⁺³,while simultaneously compensating for the charge imbalance through theintroduction of such ions as Li⁺ or Mg⁺². As an illustration, dissolvingMgAl₂ O₄ in Si₃ N₄ can result in complete charge balance. In sum, a widevariety of new materials appears to be possible based uponsilicon-aluminum-oxygen-nitrogen tetrahedra ("Sialon"), in much the sameway that the essentially infinite range of mineral silicates is built upemploying silicon-aluminum-oxygen units.

The Sialon unit cell is similar to that of beta-Si₃ N₄ (β-Si₃ N₄). Itcontains eight oxygen plus nitrogen atoms and has a range of homogeneity

    Si.sub.6-0.75x Al.sub.0.67x O.sub.x H.sub.8-x

wherein x, the number of nitrogen atoms replaced by oxygen, has amaximum of six.

An extremely significant feature of nitrogen is its ability tocoordinate tetrahedrally not only with Si⁺⁴ and Al⁺³, but also with Li⁺,the alkaline earth metals, and, possibly, Zn⁺². This is believed to bethe result of a favorable cation-to-anion size ratio for tetrahedralcoordination, shown in Table I below, coupled with the greater covalentnature of the metal-nitrogen bond, when compared to a metal-oxygen bond.

                  TABLE I                                                         ______________________________________                                        Coordination                                                                  No.                    Cation Radius:Anion Radius                             ______________________________________                                        8        Cube Corners  1-0.732                                                6        Octahedra Corners                                                                           0.732-0.414                                            4        Tetrahedra Corners                                                                          0.414-0.225                                            3        Triangle Corners                                                                            0.225-0.115                                            2        Linear        0.155-0                                                0 -                    Radius  Oxygen Radius  Nitrogen                                  Coordi-      Ratio                                                                  Ratio  Coordi-                                                Cation   Radius        R.sup.+ :RO.sup.-2                                               nation                                                                              R.sup.+ :RN.sup.-3                                                                   nation                                                 ______________________________________                                        Na.sup.+ 0.95          0.679                                                            6     0.555  6                                                      Li.sup.+ 0.60          0.429                                                            6     0.351  4                                                      Mg.sup.+2                                                                              0.65          0.464                                                            6     0.380  4                                                      Zn.sup.+2                                                                              0.75          0.536                                                            6     0.439  6, 4 (?)                                               Al.sup.+3                                                                              0.50          0.357                                                            4, 6  0.292  6, 4 (?)                                               B.sup.+3 0.20          0.143                                                            3     0.117  ?                                                      Si.sup.+4                                                                              0.41          0.293                                                            4     0.240  4                                                      Be.sup.+2                                                                              0.31          0.221                                                            3, 4  0.180  3                                                      Mn.sup.+2                                                                              0.80          0.571                                                            6     0.468  6, 4 (?)                                               Y.sup.+3 0.93          0.664                                                            6     0.544  6                                                      ______________________________________                                    

Fluorine appears to promote the crystallization of "nitrogen-mullite"and other nitride phases, possibly through a substitution of the type

    20.sup.-2 ⃡ N.sup.-3 + F.sup.-

the introduction of fluorine also appears to foster the crystallizationof nitrogen-silicate and/or nitrogen-containing crystals.

Table II records a group of approximate compositions, expressed inweight percent on the oxide basis, illustrating the parameters of theinstant invention. Since it is not known with which cation(s) thefluorine and nitrogen are combined, they are merely reported as fluoride(F) and nitrogen (N) and the oxygen ≈ fluorine and oxygen ≈ nitrogencorrection factor recited in accordance with conventional glass analysispractice. The actual batch ingredients can comprise any materials,either the oxide or other compound, which, when melted together, will beconverted to the desired oxide in the proper proportions. The fluorinewill commonly be supplied utilizing such compounds as AlF₃, MgF₂, LiF,ZnF₂, and CaF₂, depending upon the compositions involved. In thefollowing compositions, nitrogen was added as Si₃ N₄. However, othernitrogen-containing compounds, such as AlN, Si₂ ON₂, Li₃ N, or Mg₃ N₂,can be employed. Volatilization of nitrogen and fluorine from the meltis dependent upon the melting temperature and atmosphere utilized.Losses of up to 50% by weight can occur although, in general, such willrun between about 10-40%.

The batch ingredients will be compounded, ballmilled together to assistin obtaining a homogeneous melt, and the mixture then run into silica,molybdenum, or graphite crucibles. Since an inherently strongly reducingatmosphere is created by nitride, a platinum crucible cannot beutilized. And, since nitrogen solubility increases significantly in areducing atmosphere (as much as five orders of magnitude compared to anoxidizing environment), it is most beneficial to melt the compositionsin a nitrogen or other oxygen-deficient atmosphere.

Where a silica crucible is employed, the melting process will beconducted in air in a covered crucible, whereas melting in a molybdenumor graphite crucible will be undertaken under a nitrogen atmosphere.Thus, the partial pressure of oxygen (pO₂) in the melting environment isof considerable importance in fusing oxynitride compositions. Hence, ifthe environment is very reducing, i.e., pO₂ <10⁻¹⁰, volatilization ofSiO₂ as SiO takes place, as well as the vaporization of such readilyreducible oxides as ZnO and PbO. In contrast, if highly oxidizingconditions are involved, i.e., pO₂ >10⁻¹, oxidation of Si₃ N₄ in thebatch will occur.

The crucibles containing the batches were covered, placed ininduction-heated furnaces operating at about 1500°-1850° C., and thebatches melted for about 1-16 hours. The melts were poured into steelmolds to yield glass slabs and these slabs immediately transferred to anannealer commonly operating at about 600°-700° C. Samples were cut fromthe slabs to be used for the determination of physical propertymeasurements. Halides and chalcogenides may optionally be included asfining agents.

Table IIA records, in parts by weight, the actual batch materialsutilized in the examples of Table II along with the melting temperature(°C.), the type of crucible employed [SiO₂, Mo or graphite (C)], and thefurnace atmosphere employed. Where a nitrogen atmosphere was employed,the pO₂ and pN₂ are reported.

                                      TABLE II                                    __________________________________________________________________________        1  2  3  4  5  6  7  8  9  10 11 12 13 14 15 16 17                        __________________________________________________________________________    SiO.sub.2                                                                         70.4                                                                             70.4                                                                             70.4                                                                             70.4                                                                             70.4                                                                             70.4                                                                             70.4                                                                             70.4                                                                             65.4                                                                             65.4                                                                             65.4                                                                             65.4                                                                             65.4                                                                             65.4                                                                             65.4                                                                             65.4                                                                             65.4                      Al.sub.2 O.sub.3                                                                  22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                                                                             22.2                      MgO 7.0                                                                              5.0                                                                              5.0                                                                              5.0                                                                              5.0                                                                              5.0                                                                              5.0                                                                              15.0                                                                             -- -- -- 10.0                                                                             -- 7.5                                                                              7.5                                                                              --                           F   1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                                                                              1.4                       N   4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.8                       Li.sub.2 O                                                                        3.0                                                                              -- -- -- -- -- -- -- -- -- -- 5.0                                                                              5.0                                                                              5.0                                                                              -- -- --                        SrO -- 5.0                                                                              -- -- -- -- -- -- -- -- -- -- -- -- -- -- --                        BaO -- -- 5.0                                                                              -- -- -- -- -- -- -- 15.0                                                                             -- -- 10 -- 7.5                                                                              7.5                       ZnO -- -- -- 5.0                                                                              -- -- -- -- -- 15.0                                                                             -- 10.0                                                                             -- -- 7.5                                                                              -- 7.5                       Na.sub.2 O                                                                        -- -- -- -- 5.0                                                                              -- -- -- -- -- -- -- -- -- -- -- --                        K.sub.2 O                                                                         -- -- -- -- -- 5.0                                                                              -- -- -- -- -- -- -- -- -- -- --                        Y.sub.2 O.sub.3                                                                   -- -- -- -- -- -- 5.0                                                                              -- -- -- -- -- -- -- -- -- --                        Cs.sub.2 O                                                                        -- -- -- -- -- -- -- 5.0                                                                               --                                                                              -- -- -- -- -- -- -- --                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                                                                            108.8                     O≈F                                                                       -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -0.6                      O≈N                                                                       -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                                                                             -8.2                          100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                         18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34                        __________________________________________________________________________    SiO.sub.2                                                                         69.6                                                                             70.4                                                                             70.4                                                                             70.4                                                                             69.1                                                                             52.8                                                                             67.8                                                                             52.8                                                                             54.6                                                                             59.6                                                                             71.1                                                                             72.1                                                                             74.4                                                                             65.3                                                                             66.9                                                                             63.7                                                                             55.7                      Al.sub.2 O.sub.3                                                                  17.0                                                                             25.0                                                                             22.2                                                                             22.2                                                                             21.8                                                                             28.0                                                                             18.0                                                                             28.8                                                                             30.6                                                                             25.4                                                                             23.2                                                                             23.8                                                                             14.0                                                                             22.0                                                                             22.0                                                                             32.5                                                                             36.6                      F   -- 1.8                                                                              1.4                                                                              1.4                                                                              1.4                                                                              3.4                                                                              3.4                                                                              3.4                                                                              1.4                                                                              -- 1.4                                                                              2.0                                                                              -- -- -- -- --                        N   9.2                                                                              4.8                                                                              4.8                                                                              4.8                                                                              4.7                                                                              4.0                                                                              4.0                                                                              4.0                                                                              7.8                                                                              9.2                                                                              4.4                                                                              4.4                                                                              7.6                                                                              6.0                                                                              4.0                                                                              8.0                                                                              4.0                       MgO 20.9                                                                             3.9                                                                              4.0                                                                              4.0                                                                              4.9                                                                              -- -- -- 19.6                                                                             16.6                                                                             8.0                                                                              -- 17.0                                                                             17.0                                                                             14.0                                                                             -- --                        BaO -- 3.0                                                                              6.0                                                                              3.0                                                                              -- -- -- -- -- -- -- -- -- -- -- -- --                        Li.sub.2 O                                                                        -- -- -- 3.0                                                                              3.9                                                                              -- -- -- -- 5.0                                                                              -- 6.0                                                                              -- -- -- 9.5                                                                              10.6                      Y.sub.2 O.sub.3                                                                   -- -- -- -- 2.9                                                                              5.0                                                                              -- -- -- -- -- -- -- -- -- -- --                        ZnO -- -- -- -- -- 15.0                                                                             -- -- -- -- -- -- -- -- -- -- --                        MnO -- -- -- -- -- -- -- 20.0                                                                             -- -- -- -- -- -- -- -- --                        BeO -- -- -- -- -- -- 15.0                                                                             -- -- -- -- -- -- -- -- -- --                            115.7                                                                            108.9                                                                            108.8                                                                            108.8                                                                            108.7                                                                            108.2                                                                            108.2                                                                            108.2                                                                            107.9                                                                            115.8                                                                            108.1                                                                            108.3                                                                            113.0                                                                            110.3                                                                            106.9                                                                            113.7                                                                            106.9                     O≈F                                                                       -- -0.8                                                                             -0.6                                                                             -0.6                                                                             -0.6                                                                             -1.4                                                                             -1.4                                                                             -1.4                                                                             -6.6                                                                             -- -7.5                                                                             -7.5                                                                             -- -- -- -- --                        O≈N                                                                       -15.7                                                                            -8.1                                                                             -8.2                                                                             -8.2                                                                             -8.1                                                                             -6.8                                                                             -6.8                                                                             -6.8                                                                             -1.3                                                                             -15.8                                                                            -0.6                                                                             -0.8                                                                             -13.0                                                                            -10.3                                                                            -6.9                                                                             - 13.7                                                                           -6.9                          100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                                                                            100.0                          35  36  37  38   39  40  41  42   43  44  45  46                         __________________________________________________________________________    SiO.sub.2                                                                          80.3                                                                              57.9                                                                              77.8                                                                              53.0 75.7                                                                              71.4                                                                              66.2                                                                              79.8 61.9                                                                              70.4                                                                              74.3                                                                              74.4                       Al.sub.2 O.sub.3                                                                   14.6                                                                              34.0                                                                              26.5                                                                              37.6 23.2                                                                              26.7                                                                              30.8                                                                              23.8 20.6                                                                              22.2                                                                              --  21.2                       F    --  --  --  1.4  1.4 1.4 1.4 1.3  1.3 1.4 --  0.4                        N    4.0 4.0 6.0 8.3  8.3 8.3 8.3 18.7 7.8 4.3 6.0 2.0                        MgO  --  4.0 --  14.6 --  --  --  9.0  --  --  --  1.6                        Li.sub.2 O                                                                         8.0 7.0 --  --   6.3 7.1 8.2 --   --  --  20.0                                                                              4.0                        ZnO  --  --  --  --   --  --  --  --   15.5                                                                              --  --  --                         ZrO.sub.2                                                                          --  --  --  --   --  --  --  --   5.8 --  --  --                         Aa.sub.2 O.sub.5                                                                   --  --  --  --   --  --  --  --   1.0 --  --  --                         CaO  --  --  --  --   --  --  --  --   --  5.0 --  --                              106.9                                                                             106.9                                                                             110.3                                                                             114.9                                                                              114.9                                                                             114.9                                                                             114.9                                                                             132.6                                                                              113.9                                                                             108.8                                                                             110.3                                                                             103.6                      O≈F                                                                        --  --  --  -0.6 -0.6                                                                              -0.6                                                                              -0.6                                                                              -0.6 -0.5                                                                              -0.6                                                                              --  -0.2                       O≈N                                                                        -6.9                                                                              -6.9                                                                              -10.3                                                                             -14.3                                                                              -14.3                                                                             -14.3                                                                             -14.3                                                                             -32.0                                                                              -13.4                                                                             -8.2                                                                              -10.3                                                                             3.4                             100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                              100.0                                                                             100.0                                                                             100.0                                                                             100.0                                                                              100.0                                                                             100.0                                                                             100.0                                                                             100.0                      __________________________________________________________________________

                                      TABLE IIA                                   __________________________________________________________________________         1  2  3   4  5  6  7   8  9  10 11  12 13 14 15 16                       __________________________________________________________________________    SiO.sub.2                                                                          55 55 55  55 55 55 55  55 50 50 50  50 50 50 50 50                       Al.sub.2 O.sub.3                                                                   21 21 21  21 21 21 21  21 21 21 21  21 21 21 21 21                       Si.sub.3 N.sub.4                                                                   12 12 12  12 12 12 12  12 12 12 12  12 12 12 12 12                       AlF.sub.3                                                                          2  2  2   2  2  2  2   2  2  2  2   2  2  2  2  2                        MgO  7  5  5   5  5  5  5   5  15 -- --  -- 10 -- 7.5                                                                              7.5                      Li.sub.2 O                                                                         3  -- --  -- -- -- --  -- -- -- --  5  5  5  -- --                       SrO  -- -- --  -- -- -- --  -- -- -- --  -- -- -- -- --                       BaO  -- -- 5   -- -- -- --  -- -- -- 15  -- -- 10 -- 7.5                      ZnO  -- -- --  5  -- -- --  -- -- 15 --  10 -- -- 7.5                                                                              --                       Na.sub.2 O                                                                         -- -- --  -- 5  -- --  -- -- -- --  -- -- -- -- --                       K.sub.2 O                                                                          -- -- --  -- -- 5  --  -- -- -- --  -- -- -- -- --                       Y.sub.2 O.sub.3                                                                    -- -- --  -- -- -- 5   -- -- -- --  -- -- -- -- --                       Cs.sub.2 O                                                                         -- -- --  -- -- -- --  5  -- -- --  -- -- -- -- --                       BaO  -- -- --  -- -- -- --  -- -- -- --  -- -- -- -- --                       MnO  -- -- --  -- -- -- --  -- -- -- --  -- -- -- -- --                       Temp.                                                                              1650                                                                             1650                                                                             1650                                                                              1650                                                                             1650                                                                             1650                                                                             1650                                                                              1650                                                                             1650                                                                             1650                                                                             1650                                                                              1650                                                                             16750                                                                            1650                                                                             1650                                                                             1650                     Crucible                                                                           SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                         SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                         SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                         SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                pO.sub.2                                                                           air                                                                              air                                                                              air air                                                                              air                                                                              air                                                                              air air                                                                              air                                                                              air                                                                              air air                                                                              air                                                                              air                                                                              air                                                                              air                      pN.sub.2                                                                           -- -- --  -- -- -- --  -- -- -- --  -- -- -- -- --                            17  18  19  20  21  22  23  24  25  26  27  28  29                       __________________________________________________________________________    SiO.sub.2                                                                          50  40  55  55  55  53.9                                                                              40  55  38.2                                                                              29.4                                                                              29.9                                                                              57  58                       Al.sub.2 O.sub.3                                                                   21  17  25  21  21  20.6                                                                              20  15  23.9                                                                              29.4                                                                              25.4                                                                              22  22                       Si.sub.3 N.sub.4                                                                   12  23  12  12  12  11.8                                                                              10  10  9.6 19.6                                                                              23.1                                                                              11  11                       AlF.sub.3                                                                          2   --  3   2   2   2.0 5   5   4.8 2.0 --  2   3                        MgO  --  20  2   4   4   4.9 --  --  --  19.6                                                                              16.6                                                                              8   --                       Li.sub.2 O                                                                         --  --  --  --  3   3.9 --  --  --  --  5   --  6                        SrO  --  --  --  --  --  --  --  --  --  --  --  --  --                       BaO  7.5 --  3   6   3   --  15  --  --  --  --  --  --                       ZnO  7.5 --  --  --  --  --  --  --  --  --  --  --  --                       Na.sub.2 O                                                                         --  --  --  --  --  --  --  --  --  --  --  --  --                       K.sub.2 O                                                                          --  --  --  --  --  --  --  --  --  --  --  --  --                       Y.sub.2 O.sub.3                                                                    --  --  --  --  --  2.9 10  --  --  --  --  --  --                       Cs.sub.2 O                                                                         --  --  --  --  --  --  --  --  --  --  --  --  --                       BeO  --  --  --  --  --  --  --  15  --  --  --  --  --                       MnO  --  --  --  --  --  --  --  --  23.5                                                                              --  --  --  --                       Temp.                                                                              1650                                                                              1650                                                                              1800                                                                              1750                                                                              1750                                                                              1700                                                                              1650                                                                              1650                                                                              1650                                                                              1650                                                                              1700                                                                              1650                                                                              1650                     Crucible                                                                           SiO.sub.2                                                                         C   C   C   C   C   SiO.sub.2                                                                         SiO.sub.2                                                                         SiO.sub.2                                                                         SiO.sub.2                                                                         C   SiO.sub.2                                                                         SiO.sub.2                pO.sub.2                                                                           air 10.sup.-4                                                                         10.sup.-4                                                                         10.sup.-4                                                                         10.sup.-4                                                                         10.sup.-4                                                                         air air air air 10.sup.-4                                                                         air air                      pN.sub.2                                                                           --  ≈1                                                                        ≈1                                                                        ≈1                                                                        ≈1                                                                        ≈1                                                                        --  --  --  --  ≈1                                                                        --  --                            30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46                       __________________________________________________________________________    SiO.sub.2                                                                          50 46 54 38.0                                                                             42.8                                                                             67.4                                                                             45 58.8                                                                             26.8                                                                             49.0                                                                             44.7                                                                             39.5                                                                             19.7                                                                             36.9                                                                             35 65 68.0                     Al.sub.2 O.sub.3                                                                   14 22 22 32.5                                                                             36.6                                                                             14.6                                                                             34 26.5                                                                             37.1                                                                             21.9                                                                             25.4                                                                             29.5                                                                             22.6                                                                             19.4                                                                             21 -- 20.8                     Si.sub.3 N.sub.4                                                                   19 15 10 20.0                                                                             10.0                                                                             10.0                                                                             10 15.0                                                                             21.2                                                                             20.8                                                                             20.8                                                                             20.8                                                                             46.7                                                                             19.4                                                                             12 15 5.0                      AlF.sub.3                                                                          -- -- -- -- -- -- -- -- -- 2.0                                                                              2.0                                                                              2.0                                                                              2.0                                                                              2.0                                                                              2  -- 0.6                      MgO  17 17 14 -- -- -- 4  -- 14.9                                                                             -- -- -- 9.0                                                                              -- 5  -- 1.6                      Li.sub.2 O                                                                         -- -- -- 9.5                                                                              10.6                                                                             8.0                                                                              7  -- -- 6.3                                                                              7.1                                                                              8.2                                                                              -- -- -- 20 4.0                      ZnO  -- -- -- -- -- -- -- -- -- -- -- -- -- 15.5                                                                             -- -- --                       ZrO.sub.2                                                                          -- -- -- -- -- -- -- -- -- -- -- -- -- 5.8                                                                              -- -- --                       As.sub.2 O.sub.3                                                                   -- -- -- -- -- -- -- -- -- -- -- -- -- 1.0                                                                              -- -- --                       CaO  -- -- -- -- -- -- -- -- -- -- -- -- -- -- 5  -- --                       Temp.                                                                              1650                                                                             1650                                                                             1650                                                                             1650                                                                             1650                                                                             1650                                                                             1650                                                                             1850                                                                             1650                                                                             1650                                                                             1650                                                                             1650                                                                             1650                                                                             1650                                                                             1650                                                                             1500                                                                             1650                     Crucible                                                                           SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        Mo SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                                                                        SiO.sub.2                pO.sub.2                                                                           air                                                                              air                                                                              air                                                                              air                                                                              air                                                                              air                                                                              air                                                                              10.sup.-4                                                                        air                                                                              air                                                                              air                                                                              air                                                                              air                                                                              air                                                                              air                                                                              air                                                                              air                      pN.sub.2                                                                           -- -- -- -- -- -- -- ≈1                                                                       -- -- -- -- -- -- -- -- --                       __________________________________________________________________________

Example 42 formed a rather viscous melt which could not be poured intoan essentially crystal-free glass. Hence, only be melting at highertemperatures and employing severe quenching techniques could acrystal-free glass be formed. This circumstance has led to the adoptionof 17% by weight as a practical maximum N content.

Table III records various heat treatments applied to the glass bodies ofTable II to cause crystallization in situ. Thus, the glass article isheated to a temperature between about 800°-1300° C. for a period of timesufficient to develop substantial crystallization.

In the laboratory examples reported in Table II, the glass bodies werecustomarily cooled to room temperature to permit examination of glassquality. Such practice is not necessary. All that is required is thatthe molten glass batch be cooled to a temperature at least within thetransformation range and then reheated to cause crystallization. (Thetransformation range is defined as the temperature at which a moltenmass becomes an amorphous solid -- that temperature customarily beingdeemed to lie in the vicinity of the annealing point of the glass.)

The rate of crystallization is a direct function of the temperatureemployed. Therefore, a brief exposure time, e.g., 0.5 hour or less, maybe adequate to achieve substantial crystallization at the upper extremeof the crystallization range; whereas, in the cooler end of thecrystallization range, much longer periods of time i.e., up to 24 hoursor more, may be demanded.

Although temperatures as low as 800° C. can be operable, such arenormally not utilized because the rate of crystal growth is generally soslow as to be unattractive from a practical point of view. Conversely,temperatures in excess of about 1300° C. are avoided since grain growthof the crystals and deformation of the article are hazarded. Therefore,the more useful range of crystallization temperatures has been definedas about 900°-1300° C.

If desired, a two-step heat treatment procedure may e followed. Thus,the glass article will first be heated to a temperature slightly abovethe transformation range, e.g., 750°-850° C. for a sufficient length oftime, perhaps 1-8 hours, to cause extensive nucleation within the glassand initiate crystal growth. Thereafter, the nucleated article will beheated into the 900°-1300° C. range of about 2-10 hours to develop thedesired fine-grained growth of crystals.

The products of the instant invention are conventionally highlycrystalline, being greater than 50% by volume crystalline and,frequently, in excess of 75%, and the crystals are homogeneouslydispersed within the minor residual glassy matrix.

Specific dwell periods were employed in the laboratory examples reportedin Table III and such are commonly employed as a matter of conveniencein production. However, that practice is not required to obtainsatisfactory crystal growth. Rather, it is only necessary that the glassarticle be retained within the crystallization range for a sufficientperiod of time to promote extensive crystal development.

As was observed above, the growth of crystals is a function of both timeand temperature. Hence, the rate at which the glass article is heatedabove its transformation range must be carefully controlled to avoidexceeding the rate of crystal growth. Stated differently, the rate ofcrystal growth must be sufficiently rapid so as to support the bodyagainst deformation and slumping as the temperature thereof is raised toapproach or exceed the softening point of the glass. Heating rates of10° C./minute can be successfully utilized, particularly where physicalsupporting members such as formers or setters are employed to restraindeformation of the glass bodies, but heating rates of about 3°-5°C./minute have generally been found to be acceptable with allcompositions within the operable ranges. Thus, those heating rates haveproduced crystalline articles exhibiting little, if any, physicaldeformation over the full range of compositions constituting theinvention. In each of the recited heat treatments in Table III, thetemperature of an electric furnace was raised at a rate of about 5°C./minute to the listed dwell temperature and the crystallized articlespermitted to cool to room temperature at furnace rate. Hence, after theheat treatment had been completed, the crystallized articles were simplyleft within the furnace, the electric current thereto cut off, and thearticles allowed to cool to room temperature therein. This rate ofcooling was estimated to range about 3°-5° C./minute.

Table III also records a visual description of the final crystallizedproduct, an identification of the crystal phases present as determinedthrough X-ray diffraction analysis, Knoop hardness measurements (100g),and approximate coefficient of thermal expansion measurements (×10⁻⁷ /°C.) over the range of 0°-800° C. where made.

As has been explained above, an oxynitride phase, which has beendenominated "nitrogen-mullite", has been observed which has anapproximate stoichiometry of Al₃ Si₂ O₇ N. The morphology of thesecrystals is lath-like or blade-like with fairly high aspect ratios,i.e., up to about 10:1. This type of morphology renders the crystalsuseful in reinforcing an otherwise weak glass-ceramic matrix. Alsoobserved were Si₂ ON₂ and β'-Si₃ N₄ which appear to be solid solutionscontaining Li₂ O, MgO, ZnO, and Al₂ O₃. For example, Si₂ ON₂ is known tobe isostructural with LiAlSiN₂ O when Si⁺⁴ ⃡ Li⁺ + Al⁺³ substitutionoccurs. β'-Si₃ N₄ is a solid solution of the type M_(x) (Si,Al)₃ (O,N)₄where M = Li, Mg, Zn, etc. β'-Si₃ N₄ is of special interest because itis isostructural with β-Si₃ N₄ which is well-known for its highstrength, hardness, refractoriness, elastic modulus, and thermal shockresistance.

The silicate phases reported in Table III exhibited an X-ray diffractionpattern conventionally seen with such phases. However, as was observedabove, chemical analysis has indicated the presence of nitrogen in thecrystal phase. However, the manner in which the nitrogen is includedwithin the silicate structure has not been completely determined. It hasbeen postulated that the nitrogen actually forms part of the crystallattice and is not present simply as an interstitial element.

FIG. 1 is an electron micrograph of Example 38, which contains about60-65% by volume β'-Si₃ N₄, + "nitrogen-mullite", resulting from a heattreatment of 2 hours at 800° C. followed by two hours at 1000° C. FIG. 2is an electron micrograph of Example 10, illustrating interconnectedplatelets of "nitrogen-mullite" resulting from a heat treatment of fourhours at 1200° C. FIG. 3 is an electron micrograph of Example 46demonstrating a very highly crystalline body wherein beta-spodumenesolid solution constitutes the predominant crystal phase. The precursorthermally crystallizable glass was subjected to a heat treatment for 2hours at 800° C. followed by four hours at 1000° C. This glass containedless than 3.5% nitrogen such that no nitride-containing solid solutionas such was observed or identified.

                                      TABLE III                                   __________________________________________________________________________    Example                                       Knoop                                                                              Exp.                       No.  Heat Treatment                                                                            Crystal Phases   Visual Description                                                                        Hardness                                                                           Coef.                      __________________________________________________________________________    1    800° C. for two hours                                                              Beta-Quartz solid solution                                                                     Opaque, gray, fine-                                                                       636  21.0                            1000° C. for four hours                                                                             grained                                     2    1200° C. for four hours                                                            Nitrogen-mullite solid                                                                         "           666  47.2                                        solution                                                     3    800° C. for four hours                                                             Nitrogen-mullite solid solu-                                                                   "           --   44.3                            1100° C. for four hours                                                            tion, beta-quartz solid solution                             4    1100° C. for four hours                                                            "                "           --   36.4                       5    1200° C. for four hours                                                            Nitrogen-mullite solid solution                                                                "           611  60.3                       6    800° C. for four hours                                                             "                "           666  44.5                            1100° C. for four hours                                           7    800° C. for four hours                                                             "                "           --   31.2                            1100° C. for four hours                                           8    800° C. for four hours                                                             "                "           --   36.9                            1100° C. for four hours                                           9    800° C. for two hours                                                              Cordierite       "           817  41.6                            1000° C. for four hours                                           10   1200° C. for four hours                                                            Nitrogen-mullite solid solution                                                                "           718  31.6                       11   1200° C. for four hours                                                            Nitrogen-mullite solid solu-                                                                   "           704  42.5                                        tion, beta-quartz solid solution                             12   1200° C. for four hours                                                            Beta-spodumene solid solution                                                                  "           569  34.0                       13   800° C. for two hours                                                              Beta-spodumene solid solution,                                                                 "           616  28.0                            1100° C. for four hours                                                            beta-quartz solid solution                                   14   800° C. for two hours                                                              Beta-quartz solid solution                                                                     "           561  39.0                            1100° C. for two hours                                            15   1200° C. for four hours                                                            Nitrogen-mullite solid solution,                                                               "           732  50                                          cordierite, beta-quartz solid                                                 solution                                                     16   800° C. for two hours                                                              Beta-quartz solid solution                                                                     "           647  47.0                            1100° C. for two hours                                            17   800° C. for four hours                                                             Nitrogen-mullite solid solution,                                                               "           684  37.0                            1200° C. for four hours                                                            beta-quartz solid solution                                   18   1100° C. for four hours                                                            β'-Si.sub.3 N.sub.4, Si.sub.2 ON.sub.2                                                    "           --   --                         19   1100° C. for four hours                                                            Nitrogen-mullite solid solution                                                                "           --   --                         20   1100° C. for four hours                                                            "                "           --   --                         21   800° C. for two hours                                                              Beta-quartz, solid solution                                                                    "           --   --                              1100° C. for two hours                                            22   800° C. for two hours                                                              Beta-spodumene solid solution,                                                                 "           --   25.0                            1100° C. for four hours                                                            beta-quartz solid solution                                   23   800° C. for two hours                                                              Mullite          "           --   39.0                            1000° C. for two hours                                            24   800° C. for two hours                                                              4BeO . 2/Al.sub.2 O.sub.3 . 6SiO.sub.2                                                         "           --   37.0                            1250° C. for four hours                                           25   800° C. for two hours                                                              Mullite, cristobalite                                                                          "           --   64.0                            1250° C. for two hours                                            26   800° C. for two hours                                                              Petalite, cordierite                                                                           "           --   34.0                            1000° C. for four hours                                           27   800° C. for two hours                                                              β'-Si.sub.3 N.sub.4, beta-spodumene                                                       "olid       --   37.0                            1000° C. for four hours                                                            solution, beta-quartz solid solution                         28   800° C. for two hours                                                              Nitrogen-mullite solid solution                                                                "           --   31.0                            1100° C. for four hours                                           29   800° C. for two hours                                                              Beta-spodumene solid solution                                                                  "           --    8.3                            1050° C. for four hours                                           30   800° C. for two hours                                                              Petalite, Si.sub.2 ON.sub.2                                                                    "           --   41.0                            1100° C. for four hours                                           31   800° C. for two hours                                                              Cordierite       Opaque, gray, coarse-                                                                     --   26.0                            1100° C. for four hours                                                                             grained                                     32   800° C. for two hours                                                              Cordierite       "           --   32.0                            1200° C. for four hours                                           33   800° C. for two hours                                                              Beta-quartz solid solution                                                                     Opaque, gray, fine-                                                                       --   10.0                            1000° C. for four hours                                                                             grained                                     34   800° C. for two hours                                                              Beta-quartz solid solution                                                                     Opaque, gray, medium-                                                                     --    4.0                            950° C. for four hours                                                                              grained                                     35   800° C. for two hours                                                              Beta-spodumene solid solution                                                                  Opaque, gray, fine-                                                                       --   24.0                            950° C. for four hours                                                             beta-quartz solid solution,                                                                    grained                                                      Si.sub.2 ON.sub.2                                            36   800° C. for two hours                                                              Beta-quartz solid solution                                                                     "           --   33.0                            1000° C. for four hours                                           37   800° C. for two hours                                                              Nitrogen-mullite solid solution                                                                "           --   --                              1200° C. for four hours                                           38   800° C. for two hours                                                              β'-Si.sub.3 N.sub.4, nitrogen-mullite                                                     "           --   40.0                            1000° C. for two hours                                                             solid solution                                               39   800° C. for two hours                                                              Si.sub.2 ON.sub.2, beta-quartz solid solution                                                  "           --   --                              900° C. for two hours                                             40   800° C. for two hours                                                              "                "           --   32.0                            1000° C. for two hours                                            41   800° C. for two hours                                                              "                "           --   24.0                            1000° C. for two hours                                            42   800° C. for two hours                                                              β'-Si.sub.3 N.sub.4                                                                       "           --   40.0                            1000° C. for two hours                                            43   800° C. for two hours                                                              Si.sub.2 ON.sub.2                                                                              "           --   --                              1000° C. for two hours                                            44   800° C. for two hours                                                              Nitrogen-mullite solid solution                                                                "           815  --                              1300° C. for two hours                                            45   800° C. for 14 hours                                                               Lithium disilicate, lithium meta-                                                              "           --   --                                          silicate                                                     46   800° C. for two hours                                                              Beta-spodumene solid solution                                                                  "           --    0.0                            1000° C. for four hours                                           __________________________________________________________________________

As is apparent from a study of Tables II and III, thenitrogen-containing glasses are self-nucleating, i.e., no additionalnucleating agents such as TiO₂, ZrO₂, or SnO₂, which have customarilybeen employed in the past for stimulating crystal growth, are necessary.

FIG. 4 is an electron micrograph of the glass-ceramic product of Example3, FIG. 5 is an electron micrograph of Example 6, and FIG. 6 is anelectron micrograph of Example 8. It is apparent from those micrographsthat the presence of such large ions as K⁺, Cs⁺, and Ba⁺² causes the"nitrogen-mullite" to crystallize in the form of spherulites.

In articles where crystal phases containing substantial proportions of Nare desired, i.e., solid solutions such as "nitrogen-mullite", Si₂ ON₂,and β'-Si₃ N₄, the inclusion of alkali metals other than Li₂ O appearsto reduce the overall crystal content of the article such that the totalthereof will preferably not exceed about 5% by weight. Where amounts ofGroup IIA and IIB modifying oxides in excess of about 15% by weight areemployed, very little, if any, of the nitride and/or oxynitridecrystallization will be developed even where high proportions of N arepresent. Instead, such phases as cordierite, hexacelsian, magnesiumpetalite, etc. will be formed. Li₂ O, either alone or with othermodifiers, appears to suppress the development of "nitrogen-mullite" infavor of Si₂ ON₂ and/or β'-Si₃ N₄ along with beta-quartz and/orbeta-spodumene solid solutions. Therefore, whereas in lower SiO₂compositions amounts up to about 12% by weight of Li₂ O may be operable,it is preferred to maintain the level thereof below about 10%. Ingeneral, the total of other extraneous compatible metal oxides will beheld below about 5% by weight as will the amount of fluorine retained inthe article.

Example 15 utilizing a top heat treatment temperature of about 1200° C.is a preferred embodiment of the invention. As is shown in FIG. 7, theglass-ceramic body is highly crystalline and contains cordierite andbeta-quartz solid solution well reinforced with "nitrogen-mullite"crystals to impart high strength.

We claim:
 1. A glass-ceramic article having a crystal content in excess of about 50% by volume, the composition of the article consisting essentially, by weight, of about 55-70%, SiO₂, 20-30% Al₂ O₃, and 3.5-15% N, wherein the predominant crystal phase consists of "nitrogen-mullite" solid solution. 